The overall goal in the Cameron lab is to elucidate and understand the signal transduction pathways that lead to, and the processes responsible for, induced resistance in plants to microbial infection, including Systemic Acquired Resistance (SAR) and Age-related Resistance (ARR), using molecular genetics, plant pathology, physiology, biochemistry, genomics and cell biology. ARR is a form of resistance that develops in mature Arabidopsis thalianaplants in response to Pseudomonas syringae pv. tomato (Pst) infection. Unlike ARR, SAR is elicited in response to certain necrotizing infections in one part of a plant resulting in production of a long distance signal and subsequent resistance to virulent infections in distant tissues. Studies in our lab demonstrate that salicylic acid (SA) accumulation in the plant cell wall is important for the ARR response and that SA may be acting as an anti-microbial agent against Pst. SA is no longer thought to be the long distance signaling molecule in SAR. Work in our lab using dir1-1, a SAR-defective mutant, indicates that DIR1, a putative lipid transfer protein, is involved in long distant signaling to establish SAR in distant tissues.

Introduction

The overall goal of research in the Cameron lab is to elucidate and understand the signal transduction pathways that lead to, and the processes responsible for, induced disease resistance responses in plants, including Systemic Acquired Resistance (SAR) and Age-related Resistance (ARR) with the long term goal of using this information to manipulate key genes in these pathways to create highly disease resistant crops.

SAR is induced by an initial “immunizing” infection in one part of the plant resulting in broad non-specific resistance throughout the plant to normally virulent pathogens. A key feature of SAR is the movement of a mobile signals via the phloem from the “immunized” or induced leaf to distant leaves where signals are perceived and the plant becomes primed or “immune”. Upon infection with a virulent pathogen, the plant responds in a resistant manner, including a burst of pathogensis-related (PR) gene expression. Salicylic acid (SA) accumulation is necessary for SAR as demonstrated by the SAR-defective phenotype of mutants deficient in SA accumulation. However, grafting experiments using tobacco strongly suggest that SA is not the mobile signal, but is instead required in the distant leaves where it activates NPR1, a key SAR regulator that interacts with transcription factors that regulate PR expression. Work in my lab has revitalized SAR long distance signaling research by identifying DIR1, a putative lipid transfer protein (LTP) as important in long distance signaling. Our work suggests that DIR1 may chaperone a lipid signal to distant leaves during SAR to alert distant leaves to prepare for future infection.

As plants age, many become more resistant to normally virulent pathogens (Age-related resistance, ARR). My lab has made significant progress in understanding ARR in Arabidopsis. We demonstrated that accumulation of SA in the intercellular space is required during ARR. SA appears to be acting as an anti-microbial agent rather than a signaling molecule as it does in SAR. ARR in mature Arabidopsis is a robust response that reduces bacterial (Pseudomonas syringae pv tomato, Pst)growth up to 100-fold compared to growth in young plants. Arabidopsis ARR also provides protection to the oomycete pathogen Hyaloperonospora parastica (Hp), suggesting that ARR may provide broad-spectrum resistance. Our working model includes a receptor that becomes activated in mature plants, such that the receptor perceives Pst or Hp and subsequently activates the ARR signaling pathway and production of intercellular anti-microbial SA.

Ongoing Research

Systemic Acquired Resistance

We demonstrated that DIR1 moves to distant leaves during SAR using our transient Agrobacterium transformation/SAR assay and an estrogen-inducible promoter:DIR1-GFP dir1-1 SAR assay. DIR1-GFP expression was restricted to one leaf by treating just ione leaf with estrogen, allowing us to detect DIR1’s movement to distant leaves using DIR1 and GFP immunoblot analysis of phloem sap. DIR1 only moves after SAR induction and work by a number of labs support the idea that DIR1 participates in a high molecular weight protein complex in the phloem. Work by graduate student Phil Carella revealed insights into the cellular route that DIR1 takes during movement to distant leaves. His work with plants that overexpress PDLP proteins (plasmodesmata localized protein) demonstrated that access to plasmodesmata during SAR is critical for DRI1 movement to distant leaves.

Now that we know DIR1 moves from induced to distant leaves during SAR, we can use DIR1 as bait to discover interacting proteins in induced leaves that acitvate it to move once SAR has been induced, as well as to DIR1 interactors in phloem and distant leaves that contribute to DIR1’s role in transducing the SAR signal.

Identifying economically effective and environmentally friendly methods of pest control for field and greenhouse crops is increasingly important to producers and consumers alike. Researchers have discovered a number of small molecules that tap into the SAR pathway to initiate disease resistance without compromising normal growth and development. These include the resistance-inducing putative SAR mobile signals azelaic acid, glycerol-3-phosphate, pipecolic acid, dehyroabietinal, and methylsalicylic acid. Moreover, small molecules like b-aminobutyric acid (BABA), folic acid and vitamin B1/2, act at other stages to induce SAR-like resistance. We have developed a cucumber-Pseudomonas SAR model to identify small molecules that protect cucumber from pathogen infection. In collaboration with plant pathologists, the ability of SAR-inducing molecules found in the lab will be tested to identify chemicals that provide effective and environmentally sustainable resistance for Ontario cucumber growers.

Age-Related Resistance

To understand what makes mature, but not young plants competent to initiate the ARR response, we examined flowering-time mutants and photoperiod-induced flowering in young plants. Graduate student Dan Wilson demonstrated that although ARR is associated with flowering, the transition to flowering is not the developmental trigger that initiates ARR competence in mature plants. Unexpectedly, our study also revealed that SVP, a transcriptional repressor of flowering, is required for ARR. Iin young plant infections, the Pseudomonas-produced toxin coronatine interferes with defense signaling leading to repression of SA biosynthesis and enhancing pathogen growth and disease. However, this does not occur in mature plants which accumulate SA in a SVP-dependent manner. Based on this, we developed a model in which plants become ARR competent leading to activation of SVP which alleviates Pseudomonas-mediated suppression of SA-dependent plant defense. Our recent experiments indicate that SVP’s role in ARR is distinct and separable from its role as a repressor of flowering. We are continuing our studies to understand what activates SVP in mature plants and how SVP controls accumulation of intercellular SA and ARR.

Significance of the Proposed Research

Demonstrating that DIR1 moves to distant leaves during SAR and creating the estrogen-SAR and Agro-SAR assays has uniquely positioned us to discover how DIR1 becomes activated for long-distance movement and how it is perceived in distant leaves to initiate priming. We have created the tools needed to reveal the significance and function of DIR1-specific motifs, identify DIR1 interactors and monitor DIR1’s cellular route during SAR. Our research will provide SAR-specific insights and also inform the LTP and long-distance signaling fields. Our studies on multiple aspects of ARR have converged, allowing us to make substantial progress in understanding the mechanics of this pathway. SVP, a transcriptional regulator of development, is a perfect candidate to regulate the onset of ARR competence, as well as to interfere with pathogen-mediated suppression of SA biosynthesis. Understanding the priming mechanisms of SAR and ARR will inform future genetic and breeding strategies to improve disease resistance in crops throughout the growing season.

Research in the Cameron Lab is funded by NSERC, CFI and McMaster University.